The present disclosure relates to energy storage systems. In particular, the present disclosure relates to fuel cell-fuel cell hybrid systems configured to capture and store excess energy generated in renewable energy power systems.
The number of power systems relying on renewable energy sources, such as solar and/or wind sources, has increased in recent years. However, due to the intermittent nature of renewable energy sources and the variable demand of users of an electrical grid, power production does not always align with power demand. This results in undesirable supply-demand gaps within the power system. For example, when availability of the renewable energy source is low, the power system may have insufficient power supply to support the current demand on the grid. In other cases, when availability of the renewable energy source is high, power supply may exceed the current demand on the grid. This excess supply risks potential overload of the grid infrastructure, leading to grid instability, reduced reliability, and poor power quality. Power systems often curtail the use of such excess renewable energy source to avoid potential overload, resulting in underutilization of available energy.
In many cases, to address gaps where demand exceeds supply, spinning reserves, such as gas turbines, are utilized. Spinning reserves are systems that are capable of providing extra generating capacity in response to fluctuations in energy production and serve to meet an increase in energy demand when supply available from the renewable energy source is insufficient. However, to provide extra power generation within the necessary response time, spinning reserves are kept continuously running so that the reserves remain at a required operating temperature to enable quick response. This constant running reduces the overall efficiency of the power capability of the reserves. In addition, due to the need for continual operation, spinning reserves typically result in higher emissions per kWh produced when compared to conventional power plants that run on fossil fuels, effectively negating the positive environmental effects of the renewable energy system.
Other systems in addressing supply-demand gaps attempt to store excess energy captured by the renewable energy system and use the stored energy to provide extra generating capacity to the power system when needed, thereby reducing instances of curtailing of renewable energy sources. One such system is a fuel cell-based system that stores excess energy in the form of hydrogen generated by a water electrolysis process, which is then converted back to power through the use of a fuel cell during times of excess demand. However, the round-trip efficiency in storing the excess energy and converting the stored energy back to power is less than ideal with the electrolysis process operating at an efficiency of around 60% to 70%, and the fuel cell system operating at an efficiency of around 60%, resulting in a total efficiency of about 36% to 42%.
Another system for storing excess energy for later use is a battery-based system. Because batteries provide a higher overall efficiency of about 80%, battery-based systems are often utilized over fuel cell-based systems. However, the energy capacity of a battery is limited compared to hydrogen storage and, thus, battery-based systems are unable to support a grid system where demand continually exceeds supply for long periods of time. In addition, the use of batteries is less cost-effective, especially in large capacity installations where the batteries are expected to be utilized only a fraction of the time during grid operations.
As penetration of renewable energy increases, the precise and efficient management of energy produced by renewable energy power systems is becoming critical. Thus, it would be advantageous to provide an energy storage system capable of high capacity energy storage and rapid response time for flexible grid support.